Optical Constants and Structural Properties of Epitaxial MoS2 Monolayers

Abstract

Two-dimensional layers of transition-metal dichalcogenides (TMDs) have been widely studied owing to their exciting potential for applications in advanced electronic and optoelectronic devices. Typically, monolayers of TMDs are produced either by mechanical exfoliation or chemical vapor deposition (CVD). While the former produces high-quality flakes with a size limited to a few micrometers, the latter gives large-area layers but with a nonuniform surface resulting from multiple defects and randomly oriented domains. The use of epitaxy growth can produce continuous, crystalline and uniform films with fewer defects. Here, we present a comprehensive study of the optical and structural properties of a single layer of MoS₂ synthesized by molecular beam epitaxy (MBE) on a sapphire substrate. For optical characterization, we performed spectroscopic ellipsometry over a broad spectral range (from 250 to 1700 nm) under variable incident angles. The structural quality was assessed by optical microscopy, atomic force microscopy, scanning electron microscopy, and Raman spectroscopy through which we were able to confirm that our sample contains a single-atomic layer of MoS₂ with a low number of defects. Raman and photoluminescence spectroscopies revealed that MBE-synthesized MoS₂ layers exhibit a two-times higher quantum yield of photoluminescence along with lower photobleaching compared to CVD-grown MoS₂, thus making it an attractive candidate for photonic applications.

Keywords: MoS2 monolayer; dielectric properties; molecular beam epitaxy; nanophotonics; optical constants; refractive index; spectroscopic ellipsometry; transition-metal dichalcogenides.

Figure 1

(a) A schematic diagram showing the concept of MBE growth monolayer MoS₂ on a sapphire substrate. (b,c) Optical image of the epitaxial MoS₂ taken in bright field (b) and dark field (c) regimes. MoS₂ covers more than 97% of the surface. (d) SEM image of the epitaxial MoS₂ revealing the high crystallinity of the samples with the characteristic crystallite size 6 ± 2 μm. (e) The AFM topography map of MoS₂ surface with a scan area of 17.5 × 10 μm². Root mean square roughness of MoS₂ is 0.5 nm in areas without defects. (f) The AFM topography map and the cross-sectional profile of the edge of epitaxial MoS₂ along the green line, giving the MoS₂ layer thickness of ~0.9 nm. The scan area was 5.5 × 2 μm².

Figure 2

Photoluminescence (a) and Raman (b) spectra of the CVD and epitaxial monolayer MoS₂ grown on sapphire substrates. The excitation wavelength was 632.8 nm (a) and 532 nm (b). Dashed and dotted lines show deconvolution of the photoluminescence spectrum into Gaussian peaks corresponding to A-exciton and defects, respectively. The Raman peak marked as “Sp” is related to the sapphire substrate.

Figure 3

XPS characterization of CVD (a,b) and epitaxially (c,d) grown MoS₂ monolayers on sapphire substrates. Decomposition of Mo3d (left) and S2p (right) core level signals into their constituents.

Figure 4

Optical properties of MBE MoS₂. Plots of the measured and calculated MBE MoS₂ ellipsometric parameters (a) Ψ and (b) Δ. (c) Optical constants (n and k) of epitaxial monolayer MoS₂ grown on a sapphire substrate from SE analysis of panels (a,b). For the tabular data, see Table A1. For comparison, we added optical constants of CVD and exfoliated MoS₂ from ref. [37,38], respectively. (d) Measured (red line) and calculated (black line) transmittance spectra of MBE MoS₂ on sapphire matching perfectly within spectrophotometer accuracy (1%) except in the 250–270 nm range, where inaccuracy approaches 2% attributed to the low signal sensitivity of our ellipsometer in that interval. The inset is a refractive index of exfoliated, MBE, and CVD MoS₂ at 750 nm.

Figure 5

Surface plasmonic resonance (SPR) sensor based on SiO₂/Au (40 nm) chip with CVD, MBE and exfoliated MoS₂. (a) The reflectance of SPR sensor for different layer numbers of CVD, MBE, and exfoliated MoS₂. (b) The dependence of the SPR sensor sensitivity on the MoS₂ number of layers. The inset is a scheme of an SPR sensor.